1. Field of the Invention
The present invention relates generally to analog-to-digital converters and, more particularly, relates to a technique for improving the accuracy of an analog-to-digital converter.
2. Description of the Related Art
Digital processing of analog signals becomes an increasingly attractive alternative to analog processing as digital hardware becomes increasingly faster, more sophisticated, and more integrated. Also, digital systems are, in general, inherently more flexible and less sensitive to time and temperature fluctuations than analog systems. As a result, a great effort has gone into the development of analog-to-digital converters (ADCs) for transforming an analog signal to a digital representation of that signal with ever-increasing accuracy, speed, and resolution.
Analog-to-digital conversion involves amplitude quantization, where an analog input signal, which may vary continuously over a finite amplitude range, is sampled at a uniform sampling rate to map the analog input signal to a finite number of discrete amplitudes. The input signal dynamic range of an ADC is divided into a specified number of possible discrete amplitudes (i.e., quantization levels), where the number of discrete amplitude levels specifies the resolution of the ADC. For example, an ADC having 2.sup.m quantization levels generates an m-bit digital output signal, where the value of m defines the resolution of the ADC.
Another important characteristic of an ADC is its linearity (or accuracy), which is a measure of the variance, from a straight line, of the ADC transfer function, i.e., the characteristic mapping of the input signal to the corresponding output signal. Non-linearity in an ADC transfer function typically results in conversion spurs, which are fictitious signals appearing in the frequency domain (e.g., during fast Fourier transform (FFT) analysis) of signals having discontinuities that are associated with deviations from a purely linear response. Most existing techniques for increasing the accuracy of an ADC rely on analog domain methods to minimize the error due to the presence of internal and external noise sources, and to maximize the accuracy and the time and temperature stability of the analog components in the ADC.
For many applications, such as telecommunications, it is important to have low noise at steady state in the absence of an input signal, and a high signal-to-noise ratio when an input signal containing one or more carriers is present at the ADC. Accordingly, it would be useful to provide an improved ADC that reduces the conversion spurs associated with non-linearity of the ADC transfer function.